Influence of Detector Size and Positioning on Near-Infrared Measurements and Iso-pathlength Point of Turbid Materials

Abstract

Measuring physical phenomena in an experimental system is commonly limited by the detector. When dealing with spatially defined behaviors, the critical parameter is the detector size. In this work, we examine near-infrared (NIR) measurements of turbid media using different size detectors at different positions. We examine cylindrical and semi-infinite scattering samples and measure their intensity distribution. An apparent crossing point between samples with different scatterings was previously discovered and named the iso-pathlength point (IPL). Monte Carlo simulations show the expected changes due to an increase in detector size or similarly as the detector’s location is distanced from the turbid element. First, the simulations show that the intensity profile changes, as well as the apparent IPL. Next, we show the average optical pathlength, and as a result, the differential pathlength factor, are mostly influenced by the detector size in the range close to the source. Experimental measurements using different size detectors at different locations validate the influence of these parameters on the intensity profiles and apparent IPL point. These findings must be considered when assessing optical parameters based on multiple scattering models. In cases such as NIR assessment of tissue oxygenation, size and location may cause false results for absorption or optical path.

Detecting Contaminants in Water Based on Full Scattering Profiles within the Single Scattering Regime

“…This implies that the IPL point allows for the separation of scattering and absorption; changes in absorption do not affect the position of the IPL point, only attenuating its intensity. , Therefore, in order to accurately determine the position of the IPL point, the absorption coefficient must be kept constant, requiring careful sample preparation . It is also important to know the sample diameter because the position of the IPL point is proportional to the optical depth, which is accounted for both by the sample diameter and the distance between the detector and the sample. , In a previous study, it was possible to isolate the absorption coefficient from the light intensity even when the scattering was unknown . This was due to the correction for the Beer–Lambert law with the IPL separation of the scattering from the absorption.…”

“…The DPF is the differential pathlength factor, dependent upon μ a , μ s , and the scattering phase function . Previous research on the IPL point has only been conducted on samples where light experiences multiple scattering events, known as the multiple scattering regime (the measured point is larger than 1/μ s ′ from the light source). − ,, For the study of contaminants in drinking water, we consider samples where light scatters only a few times, known as the single or intermediate scattering regime. The scattering in the system is determined by the concentration of insoluble particles, their size, and the geometry of the system. , Higher concentrations of insoluble particles will result in more scattering events .…”

Effect of Spatial Modulated Light on Position of Self-Calibration Point

“…In our previous publications [14] in 13mm diameter, we found the IPL position at 117 degrees. However, in our recent publication [23], we showed that a factor of 16 increment in detector size, (13mm 2 compared to 0.8mm 2 ) shifts the IPL position by 15 degrees. Moreover, a small change in the distance of the detector from the phantom's surface may explain the additional 3 degrees shift in the IPL position.…”

Optical Method for Detection and Classification of Heavy Metal Contaminants in Water Using Iso-pathlength Point Characterization

“…16,20 To address this, adjustments to the sample diameter or the distance between the detector and the sample can be made. 21 Hence, satisfying these conditions is important to ensure a consistent IPL point for each material. Previous research on the IPL point had been conducted on samples where light experiences multiple scattering events, known as the multiple scattering regime (the measured point is larger than 1/μ s ′ from the light source).…”

“…This implies that the IPL point grants the separation of scattering and absorption; changes in absorption do not affect the position of the IPL point, merely reducing its intensity. , Therefore, precisely determining the position of the IPL point involves careful sample preparation and keeping the absorption coefficient constant. Also, knowing the sample diameter is important because the IPL point position is proportional to the optical path length. , To address this, adjustments to the sample diameter or the distance between the detector and the sample can be made . Hence, satisfying these conditions is important to ensure a consistent IPL point for each material.…”